1,3-Dipolar cyclisation reactions of nitriles with sterically encumbered cyclic triphosphanes: synthesis and electronic structure of phosphorus-rich heterocycles with tunable colour

We describe the synthesis, solid state and electronic structures of a series of tunable five-membered cationic and charge-neutral inorganic heterocycles featuring a P3CN core. 1-Aza-2,3,4-triphospholenium cations [(PR)3N(H)CR']+, [1R]+ (R' = Me, Ph, 4-MeOC6H4, 4-CF3C6H4) were formed as triflate salts by the formal [3 + 2]-cyclisation reactions of strained cyclic triphosphanes (PR)3 (R = t Bu, 2,4,6-Me3C6H2 (Mes), 2,6- i Pr2C6H3 (Dipp), 2,4,6- i Pr3C6H2 (Tipp)) with nitriles R'CN in the presence of triflic acid. The corresponding neutral free bases (PR)3NCR' (2R) were readily obtained by subsequent deprotonation with NEt3. The P3CN cores in 2R show an envelope conformation typical for cyclopentenes and present as yellow to orange compounds in the solid state as well as in solution depending on both substituents R and R' in (PR)3NCR'. The P3CN cores in [1R]+ show a significant deviation from planarity with increasing steric bulk of the R groups at phosphorus, which results in a decrease in the HOMO-LUMO gap and distinct low-energy UV-Visible absorption bands. This allows access to colours spanning red, blue, indigo, and magenta. TD-DFT calculations provide valuable insight into this phenomenon and indicate an intramolecular charge-transfer from the HOMO located on the P3 framework to the N[double bond, length as m-dash]C-R'-based LUMO in the cationic species. The cations [1R]+ represent rare examples of phosphorus-rich heterocycles with tunable colour, which can be incorporated into polymers by post-polymerization modification to afford coloured polymers, which demonstrate utility as both proton and ammonia sensors.

Synthesis and Reactivity of Bis-tris(pyrazolyl)borate Lanthanide/Aluminum Heterobimetallic Trihydride Complexes

Molecular heterobimetallic hydride complexes of lanthanide (Ln) and main-group (MG) metals exhibit chemical properties unique from their monometallic counterparts and are highly reactive species, making their synthesis and isolation challenging. Herein, molecular Ln/Al heterobimetallic trihydrides [Ln(Tp)2(μ-H)2Al(H)(N″)] [2-Ln; Ln = Y, Sm, Dy, Yb; Tp = hydrotris(1-pyrazolyl)borate; N″ = N(SiMe3)2] have been synthesized by facile insertion of aminoalane [Me3N·AlH3] into the Ln-N amide bonds of [Ln(Tp)2(N″)] (1-Ln). Thus, this is a simple synthetic strategy to access a range of Ln/Al hydrides. Reactivity studies demonstrate that 2-Ln is a heterobimetallic hydride, with evidence for the cooperative nature of 2-Ln shown by the catalytic amine-borane dehydrocoupling under ambient conditions in contrast to its monomeric counterparts.

B-P Coupling: Metal Stabilized Phosphinoborate Complexes

In an effort to establish B-P coupling reactions without the use of phosphine-borane dehydrocoupling agent, we have developed a new synthetic methodology employing group 8 metal σ-borate complex [{κ3 -H,S,S'-BH2 L2 }Ru{κ3 -H,H,S-BH3 L}] (L=NC5 H4 S), 1. Treatment of 1 with chlorodiphenyl phosphine (PPh2 Cl) yielded 1,5-P,S chelated Ru-dihydridoborate species [PPh2 H{κ3 -H,H,S-BH(OH)L}Ru{κ2 -P,S-(Ph2 P)BH2 L}], 2. The insertion of phosphine moiety (PPh2 ) by the cleavage of 3c-2e σ(Ru… H-B) bonding interaction led to the formation of B-P bond. The κ2 -P,S chelated six-membered ring adopted a boat conformation in complex 2. The heterocycle is made of all different atoms, which is one of the rarest examples of heteroatomic ring systems. Theoretical outcomes demonstrated the electronic insight of B-P coupling and stabilization through transition metal. In order to explore an alternate route of B-P bond formation, we have further explored the reaction of 1 and Ru-bis(dihydridoborate) complex, 5 with secondary phosphine oxide (SPO). Although, thermolysis of 1 with diphenylphosphine oxide yielded analogous σ-borate complex 3, the similar reaction of 5 at room temperature led to the formation of novel phosphinous(III) acid incorporated Ru(σ-borate)(dihydridoborate) complex, 6. In a similar fashion, the reaction of 5 with phosphite ligand generated Ru(σ-borate)(dihydridoborate) complex, 7, which is analogous to 6.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Metal-Free B-B Dehydrocoupling Reaction of a Simple Borane Adduct: Convenient Access to a Nucleophilic Diborane(4)

The selective formation of homonuclear bonds is of key importance in synthetic chemistry. Especially, dehydrocoupling reactions are attractive as ecologically and economically friendly alternatives to established reductive bond forming reactions, since they do not require the use of stoichiometric amounts of a reducing reagent and produce only valuable dihydrogen as by-product. Here, we report on a metal-free B-B dehydrocoupling reaction that starts directly from a simple, easily accessible BH3 adduct, providing convenient access to a new nucleophilic dihydridodiborane in excellent yield. The dihydridodiborane in turn activates dihydrogen, allowing to obtain quantitatively the dideuteridodiborane from the dihydridodiborane by D2 activation. On the basis of detailed quantum-chemical calculations, the mechanism of this unprecedented reaction is elucidated. Some key points that are essential for metal-free dehydrocoupling are disclosed, paving the way for their systematic evaluation and application.

Rhodium-Mediated Dehydrogenation of Hydroboranes and Group 14 Compounds: Base-Stabilized Silylene and Germylene Complexes vs. Transmetalation

Monocarbonyl rhodium complex LRh(CO), 1, which is stabilized by a pyrrole-based bis(phosphinimine) pincer ligand (L=κ3 -NNN'=2,5-[i Pr2 P=N(4-i PrC6 H4 )]2 -N'(C4 H2 )- ), serves as a versatile platform for the dehydrogenation of group 14 substrates. Reaction with primary and secondary silanes and germanes (MesSiH3 , Et2 SiH2 , Ph2 GeH2 , t BuGeH3 ; Mes=mesityl) liberates H2 and yields base-stabilized tetrylene compounds of the form κ2 -L(CO)Rh(ER2 ) (E=Si: R=Mes, H, 2; R=Et, 5; E=Ge: R=Ph, 6; R=t Bu, H, 8). The ":ER2 " fragment in these species bridges between the rhodium center and a phosphinimine donor. Preliminary reactions between pinacol (Pin) and κ2 -L(CO)Rh(ER2 ), E=Si, Ge, indicate that such complexes can serve as silylene and germylene synthons, releasing :ER2 and catalytically generating PinER2 . In contrast, combination of complex 1 and MesGeH3 does not yield the anticipated dehydrogenation product, but rather, transmetalation similar to that observed upon reaction between 1 and 3,5-dimethylphenylborane prevails.

Commercially available organolithium compounds as effective, simple precatalysts for silicon-nitrogen heterodehydrocoupling

A family of commercially available organolithium compounds were found to effectively catalyze the heterodehydrocoupling of silanes and amines under ambient conditions. Ubiquitous nBuLi (1) was utilized as the benchmark catalyst, where an array of primary, secondary, and tertiary arylsilanes were coupled to electron-donating amines, affording aminosilanes in high conversions with short reaction times. Preliminary mechanistic analysis is consistent with a nucleophilic-type system that involves the formation of a hypervalent silicon intermediate. This work underscores the accessibility of Si-N heterodehydrocoupling, with organolithium reagents emerging as some of the most straightforward and cost-effective precatalysts for this transformation.

Zinc-Catalyzed Chemoselective Reduction of Nitriles to N-Silylimines through Hydrosilylation: Insights into the Reaction Mechanism

The N,N'-chelated conjugated bis-guanidinate (CBG) supported zinc hydride (Zn-1) pre-catalyzed highly challenging chemoselective mono-hydrosilylation of a wide range of nitriles to exclusive N-silylimines and/or N,N'-silyldiimines is reported. Furthermore, the effectiveness of pre-catalyst Zn-1 is compared with another pre-catalyst analogue, i.e., ^DiethylNacNac zinc hydride (Zn-2), to know the ligand effect. We observed that pre-catalyst Zn-1 shows high efficiency and better selectivity than pre-catalyst Zn-2 for reducing nitriles to N-silylimines. Mechanistic studies indicate the insertion of the C≡N bond of nitrile into Zn-H to form the zinc vinylidenamido complexes (Zn-1' and Zn-2'). The active catalysts Zn-1' and Zn-2' are confirmed by NMR, mass spectrometry, and single-crystal X-ray diffraction analyses. A most plausible catalytic cycle has been explored depending on stoichiometric experiments, active catalysts isolation, and in situ studies. Moreover, the synthetic utility of this protocol was demonstrated.

Access to germasiloxanes and alkynylgermanes mediated by earth-abundant species

The reactions between silanols or terminal acetylenes with alkynylgermanes have been accomplished using potassium bis(trimethylsilyl)amide as the catalyst. This strategy has provided an entry point into various organogermanes including germasiloxanes and alkynylgermanes. Remarkably, not only KHMDS but also simple bases such as KOH can serve as efficient catalysts in this process.

Sustainable preparation of aminosilane monomers, oligomers, and polymers through Si-N dehydrocoupling catalysis

This article covers historical and recent efforts to catalyse the dehydrocoupling of amines and silanes, a direct method for Si-N bond formation that offers hydrogen as a byproduct. In some applications, this transformation can be used as a sustainable replacement for traditional aminosilane synthesis, which demands corrosive chlorosilanes while generating one equivalent of ammonium salt waste for each Si-N bond that is formed. These advantages have driven the development of Si-N dehydrocoupling catalysts that span the periodic table, affording mechanistic insight that has led to advances in efficiency and selectivity. Given the divergence in precursors being used, characterization methods being relied on, and applications being targeted, this article highlights the formation of monomeric aminosilanes separately from oligomeric and polymeric aminosilanes. A recent study that allowed for the manganese catalysed synthesis of perhydropolysilazane and commercial chemical vapor deposition precursors is featured, and key opportunities for advancing the field of Si-N dehydrocoupling catalysis are discussed.

Silicon-nitrogen bond formation via dealkynative coupling of amines with bis(trimethylsilyl)acetylene mediated by KHMDS

The catalytic synthesis of silylamines mediated by s- and p-block catalysts is largely underdeveloped. Herein, commercially available potassium bis(trimethylsilyl)amide serves as an efficient alternative to transition metal complexes. N-H/Si-C dealkynative coupling was achieved by means of user-friendly main-group catalysis with ample substrate scope and high chemoselectivity.

Structurally characterised intermediate of the oxidative addition of a heteroleptic germylene to gallanediyle

Bond activation reactions using main group metal complexes are gaining increasing interest. We report on reactions of LGa (L = HC[C(Me)N(Ar)]₂, Ar = Dipp = 2,6-i-Pr₂C₆H₃,) with heteroleptic tetrylenes L'ECl (E = Ge, Sn; L' = N(SiMe₃)Ar), yielding the donor-acceptor complex LGa-Sn(Cl)L' (1) or the oxidative addition product L(Cl)GaGeL' (3). The reaction with DMPGeCl (DMP = 2,6-Mes₂C₆H₃, Mes = 2,4,6-Me₃C₆H₂) yielded LGa(μ-Cl)GeDMP (2), which represents an intermediate of the oxidative addition reaction. 1-3 were characterized by NMR and IR spectroscopy as well as by single crystal X-ray diffraction (sc-XRD), while their electronic nature was analyzed by quantum chemical calculations.

N-Silylamines in catalysis: synthesis and reactivity

A summary of the catalytic synthesis and reactivity of N-silylated amines is presented. Dehydrocoupling of amines with silanes, hydrosilylation of imines and dealkenylative coupling of amines with vinylsilanes are three ways to achieve their catalytic syntheses. The resultant N-silylamines serve as substrates in a variety of reactions, including C-N and C-C bond forming reactions, and are preferred in transformations because of the facile Si-N hydrolytic cleavage to reveal free amine products upon reaction completion. This review highlights the distinct electronic properties of N-silyl amines, N-silyl imines and N-silyl enamines that result in complementary reactivity to that of parent non-silyl variants.

Synthesis, characterisation and reactivity of group 2 complexes with a thiopyridyl scorpionate ligand

Herein we report the reactivity of the proligand tris(2-pyridylthio)methane (HTptm) with various Alkaline Earth (AE) reagents: (1) dialkylmagnesium reagents and (2) AE bis-amides (AE = Mg-Ba). Heteroleptic complexes of general formulae [Mg(Tptm)(R)] (R = Me, ⁿBu; Tptm = {C(S-C₅H₄N)₃}^-) and [AE(Tptm)(N'')] (AE = Mg-Ba; N'' = {N(SiMe₃)₂}^-) were targeted from the reaction of HTptm with R₂Mg or [AE(N'')₂]₂. Reaction of the proligand with dialkylmagnesium reagents led to formation of [{Mg(κ³C,N,N-C{Bu}{S-C₅H₄N}₂)(μ-S-C₅H₄N)}₂] (1) and [{Mg(κ³C,N,N-C{Me}{S-C₅H₄N}₂)(μ-OSiMe₃)}₂] (2) respectively, as a result of a novel transfer of an alkyl group onto the methanide carbon with concomitant C-S bond cleavage. However, reactivity of bis-amide precursors for Mg and Ca did afford the target species [AE(Tptm)(N'')] (3-AE; AE = Mg-Ca), although these proved susceptible to ligand degradation processes. DFT calculations show that alkyl transfer in the putative [Mg(Tptm)(ⁿBu)] (1m') system and amide transfer in 3-Ca is a facile process that induces C-S bond cleavage in the Tptm ligand. 3-Mg and 3-Ca were also tested as catalysts for the hydrophosphination of selected alkenes and alkynes, including the first example of mono-hydrophosphination of 4-ethynylpyridine which was achieved with high conversions and excellent regio- and stereochemical control.

Calix[4]pyrrolato Aluminate Catalyzes the Dehydrocoupling of Phenylphosphine Borane to High Molar Weight Polymers

High molar weight polyphosphinoboranes represent materials with auspicious properties, but their preparation requires transition metal‐based catalysts. Here, calix[4]pyrrolato aluminate is shown to induce the dehydropolymerization of phosphine boranes to high molar mass polyphosphinoboranes (up to M_n=43 000 Da). Combined GPC and ³¹P DOSY NMR spectroscopic analyses, quantum chemical computations, and stoichiometric reactions disclose a P−H bond activation by the cooperative action of the square‐planar aluminate and the electron‐rich ligand framework. This first transition metal‐free catalyst for P−B dehydrocoupling overcomes the problem of residual d‐block metal impurities in the resulting polymers that might interfere with the reproducibility of the properties for this emerging class of inorganic materials.

Isolable Tin(II) Hydrides Featuring Sterically Undemanding Pincer-Type Ligands and Their Dehydrogenative Sn-Sn Bond Forming Reactions to Distannynes Promoted by Lewis Acidic Tri-sec-butylborane

Unlike isolable tin(II) hydrides supported by bulky ligands reported in the literature, this research describes the synthesis and characterization of thermally stable tin(II) hydrides L^PhSnH (1-H) and ^MeLSnH (2-H) stabilized by sterically undemanding N,N,N-coordinating pincer-type ligands (L^Ph = 2,5-dipyridyl-3,4-diphenylpyrrolato; ^MeL = 2,5-bis(6-methylpyridyl)pyrrolato). The results from previous reports reveal that attempts to access tin(II) hydrides containing less-bulky ligands have had limited success, and decomposition to tin(I) distannynes often occurs. The key to the successful isolation of 1-H and 2-H is the identification of the role of Lewis acidic B^sBu₃, generated upon delivering hydride from commonly used hydride reagents M[B^sBu₃H] ("selectrides", M = Li or K). This study details compelling experimental evidence and theoretical results of the role played by B^sBu₃, which catalyzes the dehydrocoupling reactions of 1-H and 2-H to yield tin(I) distannynes L^PhSn-SnL^Ph (1₂) and ^MeLSn-Sn^MeL (2₂) with the liberation of H₂. To avoid the interference of B^sBu₃, 1-H and 2-H can be isolated in pure forms using pinacolborane as the hydride donor with L^PhSnOMe (1-OMe) and ^MeLSnOMe (2-OMe) as reactants, respectively. DFT calculations and experimental observations indicate that the coordination of the Sn-H bond of 1-H to B^sBu₃ leaves an electrophilic tin center, rendering the nucleophilic attack by the second equivalent of 1-H forming a Sn-Sn bond.

Selective Benzylic CH-Borylations by Tandem Cobalt Catalysis

Metal‐catalyzed C−H activations are environmentally and economically attractive synthetic strategies for the construction of functional molecules as they obviate the need for pre‐functionalized substrates and minimize waste generation. Great challenges reside in the control of selectivities, the utilization of unbiased hydrocarbons, and the operation of atom‐economical dehydrocoupling mechanisms. An especially mild borylation of benzylic CH bonds was developed with the ligand‐free pre‐catalyst Co[N(SiMe₃)₂]₂ and the bench‐stable and inexpensive borylation reagent B₂pin₂ that produces H₂ as the only by‐product. A full set of kinetic, spectroscopic, and preparative mechanistic studies are indicative of a tandem catalysis mechanism of CH‐borylation and dehydrocoupling via molecular Co^I catalysts.

Inorganometallics (Transition Metal-Metalloid Complexes) and Catalysis

While the formation and breaking of transition metal (TM)–carbon bonds plays a pivotal role in the catalysis of organic compounds, the reactivity of inorganometallic species, that is, those involving the transition metal (TM)–metalloid (E) bond, is of key importance in most conversions of metalloid derivatives catalyzed by TM complexes. This Review presents the background of inorganometallic catalysis and its development over the last 15 years. The results of mechanistic studies presented in the Review are related to the occurrence of TM–E and TM–H compounds as reactive intermediates in the catalytic transformations of selected metalloids (E = B, Si, Ge, Sn, As, Sb, or Te). The Review illustrates the significance of inorganometallics in catalysis of the following processes: addition of metalloid–hydrogen and metalloid–metalloid bonds to unsaturated compounds; activation and functionalization of C–H bonds and C–X bonds with hydrometalloids and bismetalloids; activation and functionalization of C–H bonds with vinylmetalloids, metalloid halides, and sulfonates; and dehydrocoupling of hydrometalloids. This first Review on inorganometallic catalysis sums up the developments in the catalytic methods for the synthesis of organometalloid compounds and their applications in advanced organic synthesis as a part of tandem reactions.

Atypical and Asymmetric 1,3-P,N Ligands: Synthesis, Coordination and Catalytic Performance of Cycloiminophosphanes

Novel seven-membered cyclic imine-based 1,3-P,N ligands were obtained by capturing a Beckmann nitrilium ion intermediate generated in situ from cyclohexanone with benzotriazole, and then displacing it by a secondary phosphane under triflic acid promotion. These "cycloiminophosphanes" possess flexible non-isomerizable tetrahydroazepine rings with a high basicity; this sets them apart from previously reported iminophophanes. The donor strength of the ligands was investigated by using their P-κ1 - and P,N-κ2 -tungsten(0) carbonyl complexes, by determining the IR frequency of the trans-CO ligands. Complexes with [RhCp*Cl2 ]2 demonstrated the hemilability of the ligands, giving a dynamic equilibrium of κ1 and κ2 species; treatment with AgOTf gives full conversion to the κ2 complex. The potential for catalysis was shown in the RuII -catalyzed, solvent-free hydration of benzonitrile and the RuII - and IrI -catalyzed transfer hydrogenation of cyclohexanone in isopropanol. Finally, to enable access to asymmetric catalysts, chiral cycloiminophosphanes were prepared from l-menthone, as well as their P,N-κ2 -RhIII and a P-κ1 -RuII complexes.

Reactivity of a Phosphinoamidinate-Stabilized Disilylene toward H-X Bonds

An efficient method for the preparation of a phosphinoamidinate-supported disilylene was developed, and its reactivity toward H-E bonds (E = elements from Groups 13-15) was studied. With HBpin, transfer of the ligand from silicon to boron was observed to afford (NP)Bpin. Reaction with a silane (H₃SiPh) took place only at elevated temperatures, at which point oxidative addition of the N-P bond of the NP-ligand to one of the silicon atoms of the disilylene occurred prior to Si-H addition involving the remaining silylene center. In contrast, reaction of the disilylene with phosphine, HPPh₂ furnished the phosphidosilylene (NP)SiPPh₂ (16) together with a highly transient species that, on the basis of a trapping experiment with H₃SiPh, is proposed to be the hydridosilylene (NP)SiH, 17. Interestingly, 16 reacts with HPPh₂ to give the diphosphine (PPh₂)₂, most likely via a direct σ-bond metathesis process. The aforementioned products have been characterized by multinuclear NMR and single-crystal X-ray diffraction studies.

Aluminum Amidinates: Insights into Alkyne Hydroboration

The mechanism of the aluminum-mediated hydroboration of terminal alkynes was investigated using a series of novel aluminum amidinate hydride and alkyl complexes bearing symmetric and asymmetric ligands. The new aluminum complexes were fully characterized and found to facilitate the formation of the (E)-vinylboronate hydroboration product, with rates and orders of reaction linked to complex size and stability. Kinetic analysis and stoichiometric reactions were used to elucidate the mechanism, which we propose to proceed via the initial formation of an Al-borane adduct. Additionally, the most unstable complex was found to promote decomposition of the pinacolborane substrate to borane (BH3), which can then proceed to catalyze the reaction. This mechanism is in contrast to previously reported aluminum hydride-catalyzed hydroboration reactions, which are proposed to proceed via the initial formation of an aluminum acetylide, or by hydroalumination to form a vinylboronate ester as the first step in the catalytic cycle.

Metal-Free Bond Activation by Carboranyl Diphosphines

We report metal-free bond activation by the carboranyl diphosphine 1-P^tBu₂-2-PⁱPr₂-C₂B₁₀H₁₀. This main group element system contains basic binding sites and possesses the ability to cycle through two-electron redox states. The reported reactions with selected main group hydrides and alcohols occur via the formal oxidation of the phosphine groups and concomitant reduction of the boron cage. These transformations, which are driven by the cooperation between the electron-donating exohedral substituents and the electron-accepting cluster, differ from those of "regular" phosphines and are reminiscent of oxidative addition to transition metal centers, thus representing a new approach to metal-free bond activation.

Catalytic and non-catalytic hydroboration of carbonyls: quantum-chemical studies

The addition of hydroboranes across several unsaturated moieties is a universal synthetic tool for the reduction or functionalization of unsaturated moieties. Given the sustainable nature of this process, the development of more environmentally-benign approaches (main-group catalysis or uncatalysed approaches) for hydroboration has gained considerable recent momentum. The present paper examines both catalyst-free and KF-mediated hydroboration of carbonyl compounds with the use of quantum-chemical methods. The results of computations for several potential reaction pathways are juxtaposed with experiment-based calculations, which leads to stepwise mechanisms and energy profiles for the reactions of pinacolborane with benzaldehyde and acetophenone (in the presence of KF). For each step of these reactions, we provide an accurate description of the geometric and electronic structures of corresponding stationary points. Five different levels of theory are employed to select the most applicable theoretical approach and develop a computational protocol for further research. Upon selection of the best-performing methods, larger molecular systems are studied to explore possible more complex pathways at the M06-2X/6-311++G(2d,p) and ωB97XD/6-311++G(2d,p) levels of theory, which brings up multi-pathway, overlapping catalytic cycles. The mechanism of solvent-free, catalyst-free hydroboration of aldehydes is also revisited through the prism of the elaborated methodology, which leads to a whole new perspective on the pathways of this and similar reactions, with a multimolecular cascade of hydride transfers being more energetically favoured than a four-membered transition state.

Mechanistic insights into dehydrocoupling of amine boranes using dinuclear zirconocene complexes

Catalytic dehydrocoupling of H₃B·NMe₂H using Cp₂Zr(Cl)(μ-Me₃SiC₃SiMe₃)Zr(Cl)Cp₂ (1)/MeLi was studied. Spectroscopic monitoring and stoichiometric experiments show the formation and interconversion of several catalytically active Zr species.

Cleavage of the Inert C(sp2)-Ar σ-Bond of Alkenes by a Spatial Constrained Interaction with Phosphinidene

[1 + 2] cycloaddition is a classical reaction between the electrophilic phosphinidene and an alkene. However, a spatial constraint blocks this well-known reaction and enables an unprecedented chemoselective C(sp²)-Ar σ-bond insertion of the alkene. The theoretical calculations demonstrate that this C-C bond cleavage is energetically feasible and thermodynamically favored through an electrophilic rearrangement and concomitant 1,9-aryl migration without involving any strained intermediate.

Visible-Light-Promoted Photoredox Dehydrogenative Coupling of Phosphines and Thiophenols

Herein, by applying visible-light photoredox catalysis, we have now achieved the first example of catalytic dehydrogenative coupling of phosphines and thiophenols that proceeds at room temperature. Key to our success is the use of benzaldehyde as a soft oxidant, which avoids the issue of phosphine oxidation. Furthermore, we observed the unexpected dealkylative coupling of secondary and tertiary alkylphosphine with thiophenols.

Dichotomous Si-H Bond Activation by Alkoxide and Alcohol in Base-Catalyzed Dehydrocoupling of Silanes

The activation of silanes in dehydrogenative coupling with alcohols under general base catalysis was studied experimentally (using multinuclear NMR, IR, and UV-visible spectroscopies) and computationally (at DFT M06/6-311++G(d,p) theory level) on the example of Ph_4-nSiH_n (n = 1-3) interaction with (CF₃)₂CHOH in the presence of Et₃N. The effect of the phenyl groups' number and H^- substitution by the electron-withdrawing (CF₃)₂CHO^- group on Si-H bond hydricity (quantified as hydride-donating ability, HDA) and Lewis acidity of silicon atom (characterized by maxima of molecular electrostatic potential) was accessed. Our results show the coordination of Lewis base (Y = Me₃N, ROH, OR^-) leads to the increased hydricity of pentacoordinate hypervalent Ph_4-nSi(Y)H_n complexes and a decrease of the reaction barrier for H₂ release. The formation of tertiary complexes [Ph_4-nSi(Y)H_n]···HOR is a critical prerequisite for the dehydrocoupling with alkoxides being ideal activators. The latter can be external or internal, generated by in situ HOR deprotonation. The mutual effect of tetrel interaction and dihydrogen bonding in tertiary complexes (RO^-)Ph_4-nSiH_n···HOR leads to dichotomous activation of Si-H bond promoting the proton-hydride transfer and H₂ release.

Syntheses of Dianionic α-Iminopyridine Rare-Earth Metal Complexes and Their Catalytic Acitivities toward Dehydrogenative Coupling of Amines with Hydrosilanes

Reactions of [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ with aminomethylene-substituted pyridine 2-[O(CH₂CH₂)₂NCH₂CH₂NCH₂]C₅H₄N (1) gave the dianionic α-iminopyridine rare-earth metal amido complexes {μ-η²:σ¹:κ¹:κ¹-2-[O(CH₂CH₂)₂NCH₂CH₂NCH]C₅H₄N}₂RE₂[N(SiMe₃)₂]₂ (RE = Y(2a), La(2b), Pr(2c), Nd(2d), Sm(2e), Dy(2f), Er(2g), and Lu (2h)). However, reaction of [(Me₃Si)₂N]₃Y(μ-Cl)Li(THF)₃ with pyridin-2-ylmethyl-substituted amines such as 2-(RNHCH₂)C₅H₄N (R = ^tBu (3a) and 2,6-ⁱPr₂Ph (3b)) or benzyl-substituted amine O(CH₂CH₂)₂NCH₂CH₂NHCH₂C₆H₅ (5) afforded the corresponding yttrium complexes containing monoanionic ligands [2-(RNCH₂)C₅H₄N]₂YN(SiMe₃)₂ (R = ^tBu (4a) and 2,6-ⁱPr₂Ph (4b)) or [O(CH₂CH₂)₂NCH₂CH₂NCH₂C₆H₅][(Me₃Si)₂N)]Y(μ-Cl)(μ-η³-O(CH₂CH₂)₂NCH₂CH₂NCH₂C₆H₅)Li(THF) (6). Dianionic α-iminopyridine rare-earth metal amido complexes showed high catalytic activities for the dehydrogenation coupling reaction of hydrosilanes and amines providing a variety of silylamines in high yields.

Phosphorus-ylides: powerful substituents for the stabilization of reactive main group compounds

Phosphorus ylides are 1,2-dipolar compounds with a negative charge on the carbon atom. This charge is stabilized by the neighbouring onium moiety, but can also be shifted towards other substituents thus making ylides strong π donor ligands and hence ideal substituents to stabilize reactive compounds such as cations and low-valent main group species. Furthermore, the donor strength and the steric properties can easily be tuned to meet different requirements for stabilizing reactive compounds and for tailoring the properties and reactivities of the main group element. Although the use of ylide substituents in main group chemistry is still in its infancy, the first examples of isolated compounds impressively demonstrate the potential of these ligands. This review summarizes the most important discoveries also in comparison to other substituents, thus outlining avenues for future research directions.

Magnesocenophane-Catalyzed Amine Borane Dehydrocoupling

The Lewis acidities of a series of [n]magnesocenophanes (1 a–d) have been investigated computationally and found to be a function of the tilt of the cyclopentadienyl moieties. Their catalytic abilities in amine borane dehydrogenation/dehydrocoupling reactions have been probed, and C[1]magnesocenophane (1 a) has been shown to effectively catalyze the dehydrogenation/dehydrocoupling of dimethylamine borane (2 a) and diisopropylamine borane (2 b) under ambient conditions. Furthermore, the mechanism of the reaction with 2 a has been investigated experimentally and computationally, and the results imply a ligand‐assisted mechanism involving stepwise proton and hydride transfer, with dimethylaminoborane as the key intermediate.

Dialumenes - aryl vs. silyl stabilisation for small molecule activation and catalysis

Main group multiple bonds have proven their ability to act as transition metal mimics in the last few decades. However, catalytic application of these species is still in its infancy. Herein we report the second neutral NHC-stabilised dialumene species by use of a supporting aryl ligand (3). Different to the trans-planar silyl-substituted dialumene (3Si), compound 3 features a trans-bent and twisted geometry. The differences between the two dialumenes are explored computationally (using B3LYP-D3/6-311G(d)) as well as experimentally. A high influence of the ligand's steric demand on the structural motif is revealed, giving rise to enhanced reactivity of 3 enabled by a higher flexibility in addition to different polarisation of the aluminium centres. As such, facile activation of dihydrogen is now achievable. The influence of ligand choice is further implicated in two different catalytic reactions; not only is the aryl-stabilised dialumene more catalytically active but the resulting product distributions also differ, thus indicating the likelihood of alternate mechanisms simply through a change of supporting ligand.

Ligand controlled reactivity: a trans-bent and twisted geometry enables dihydrogen activation and enhanced catalytic activity for NHC-stabilised dialumenes.

Dehydrocoupling - an alternative approach to functionalizing germanium nanoparticle surfaces

Surface functionalization is an essential aspect of nanoparticle design and preparation; it can impart stability, processability, functionality, as well as tailor optoelectronic properties that facilitate future applications. Herein we report a new approach toward modifying germanium nanoparticle (GeNP) surfaces and for the first time tether alkyl chains to the NP surfaces through Si-Ge bonds. This was achieved via heteronuclear dehydrocoupling reactions involving alkylsilanes and Ge-H moieties on the NP surfaces. The resulting solution processable RR'2Si-GeNPs (R = octadecyl or PDMS; R' = H or CH3) were characterized using FTIR, Raman, 1H-NMR, XRD, TEM, HAADF, and EELS and were found to retain the crystallinity of the parent GeNP platform.

Heavier Alkaline-Earth Catalyzed Dehydrocoupling of Silanes and Alcohols for the Synthesis of Metallo-Polysilylethers

The dehydrocoupling of silanes and alcohols mediated by heavier alkaline-earth catalysts, [Ae{N(SiMe₃ )₂ }₂ ⋅(THF)₂ ] (I-III) and [Ae{CH(SiMe₃ )₂ }₂ ⋅(THF)₂ ], (IV-VI) (Ae=Ca, Sr, Ba) is described. Primary, secondary, and tertiary alcohols were coupled to phenylsilane or diphenylsilane, whereas tertiary silanes are less tolerant towards bulky substrates. Some control over reaction selectivity towards mono-, di-, or tri-substituted silylether products was achieved through alteration of reaction stoichiometry, conditions, and catalyst. The ferrocenyl silylether, FeCp(C₅ H₄ SiPh(OBn)₂ ) (2), was prepared and fully characterized from the ferrocenylsilane, FeCp(C₅ H₄ SiPhH₂ ) (1), and benzyl alcohol using barium catalysis. Stoichiometric experiments suggested a reaction manifold involving the formation of Ae-alkoxide and hydride species, and a series of dimeric Ae-alkoxides [(Ph₃ CO)Ae(μ₂ -OCPh₃ )Ae(THF)] (3 a-c, Ae=Ca, Sr, Ba) were isolated and fully characterized. Mechanistic experiments suggested a complex reaction mechanism involving dimeric or polynuclear active species, whose kinetics are highly dependent on variables such as the identity and concentration of the precatalyst, silane, and alcohol. Turnover frequencies increase on descending Group 2 of the periodic table, with the barium precatalyst III displaying an apparent first-order dependence in both silane and alcohol, and an optimum catalyst loading of 3 mol % Ba, above which activity decreases. With precatalyst III in THF, ferrocene-containing poly- and oligosilylethers with ferrocene pendent to- (P1-P4) or as a constituent (P5, P6) of the main polymer chain were prepared from 1 or Fe(C₅ H₄ SiPhH₂ )₂ (4) with diols 1,4-(HOCH₂ )₂ -(C₆ H₄ ) and 1,4-(CH(CH₃ )OH)₂ -(C₆ H₄ ), respectively. The resultant materials were characterized by NMR spectroscopy, gel permeation chromatography (GPC) and DOSY NMR spectroscopy, with estimated molecular weights in excess of 20,000 Da for P1 and P4. The iron centers display reversible redox behavior and thermal analysis showed P1 and P5 to be promising precursors to magnetic ceramic materials.

Ammonia borane as a reducing agent in organic synthesis

Ammonia borane is gaining increasing attention as a sustainable and atom-economical winning reagent for the reduction of several substrates.

Half-sandwich ruthenium(ii) complexes with tethered arene-phosphinite ligands: synthesis, structure and application in catalytic cross dehydrogenative coupling reactions of silanes and alcohols

The preparation of the tethered arene-ruthenium(ii) complexes [RuCl₂{η⁶:κ¹(P)-C₆H₅(CH₂)_nOPR₂}] (R = Ph, n = 1 (9a), 2 (9b), 3 (9c); R = ⁱPr, n = 1 (10a), 2 (10b), 3 (10c)) from the corresponding phosphinite ligands R₂PO(CH₂)_nPh (R = Ph, n = 1 (1a), 2 (1b), 3 (1c); R = ⁱPr, n = 1 (2a), 2 (2b), 3 (2c)) is presented. Thus, in a first step, the treatment at room temperature of tetrahydrofuran solutions of dimers [{RuCl(μ-Cl)(η⁶-arene)}₂] (arene = p-cymene (3), benzene (4)) with 1-2a-c led to the clean formation of the corresponding mononuclear derivatives [RuCl₂(η⁶-p-cymene){R₂PO(CH₂)_nPh}] (5-6a-c) and [RuCl₂(η⁶-benzene){R₂PO(CH₂)_nPh}] (7-8a-c), which were isolated in 66-99% yield. The subsequent heating of 1,2-dichloroethane solutions of these compounds at 120 °C allowed the exchange of the coordinated arene. The substitution process proceeded faster with the benzene derivatives 7-8a-c, from which complexes 9-10a-c were generated in 61-82% yield after 0.5-10 h of heating. The molecular structures of [RuCl₂(η⁶-p-cymene){ⁱPr₂PO(CH₂)₃Ph}] (6c) and [RuCl₂{η⁶:κ¹(P)-C₆H₅(CH₂)_nOPⁱPr₂}] (n = 1 (10a), 2 (10b), 3 (10c)) were unequivocally confirmed by X-ray diffraction methods. In addition, complexes [RuCl₂{η⁶:κ¹(P)-C₆H₅(CH₂)_nOPR₂}] (9-10a-c) proved to be active catalysts for the dehydrogenative coupling of hydrosilanes and alcohols under mild conditions (r.t.). The best results were obtained with [RuCl₂{η⁶:κ¹(P)-C₆H₅(CH₂)₃OPⁱPr₂}] (10c), which reached TOF and TON values up to 117 600 h^-1 and 57 000, respectively.

Steric Effects Dictate the Formation of Terminal Arylborylene Complexes of Ruthenium from Dihydroboranes

The steric and electronic properties of aryl substituents in monoaryl borohydrides (Li[ArBH₃ ]) and dihydroboranes were systematically varied and their reactions with [Ru(PCy₃ )₂ HCl(H₂ )] (Cy: cyclohexyl) were studied, resulting in bis(σ)-borane or terminal borylene complexes of ruthenium. These variations allowed for the investigation of the factors involved in the activation of dihydroboranes in the synthesis of terminal borylene complexes. The complexes were studied by multinuclear NMR spectroscopy, mass spectrometry, X-ray diffraction analysis, and density functional theory (DFT) calculations. The experimental and computational results suggest that the ortho-substitution of the aryl groups is necessary for the formation of terminal borylene complexes.